1. Field of the Invention
The invention relates to a method for setting at least one predetermined distance between a machine tool and a metallic or non-metallic, electrically conductive workpiece.
2. The Prior Art
In the case of many machine tools, such as cutting torch tools, which serve for machining an electrically conductive, preferably metallic workpiece, it is of great importance that a predetermined distance from the workpiece is adjusted as precisely as possible, for the machining process. If, for example, the distance between a plasma cutting torch and the workpiece to be cut is not adjusted precisely, this results in increased wear of the plasma cutting torch and imprecise machining of the workpiece. A precise adjustment of the distance at which the plasma cutting torch is ignited, i.e. of the distance by which the burner is removed from the workpiece during the cutting procedure, is difficult, in particular, because it is frequently not possible to precisely determine the position of the workpiece, i.e. its height. A known method for adjusting the ignition distance consists in moving the cutting torch toward the workpiece until it touches the latter. Subsequently, it is lifted from the workpiece by means of a mechanical path measurement device, up to the ignition distance. This method has the disadvantage, however, that it is imprecise if the workpiece is a thin sheet metal. The sheet metal is deformed when the cutting torch impacts it, and the latter penetrates into the indentations in the sheet metal, so that the distance between cutting torch and the surface of the sheet metal, from which the cutting torch is lifted to the ignition distance by means of the path measurement device, is not zero but rather negative. Furthermore, the deformation of the sheet metal is damage that is frequently not desired.
Another known method provides that the torch is first moved towards the workpiece, but then is stopped at a predetermined distance above the workpiece. This distance is called the first location distance. From the first location distance, the torch is brought to the desired ignition distance by means of the path measurement device. The accuracy of the ignition distance that is set therefore depends on the accuracy with which the first location distance is set. For setting the first location distance, the torch has a ring-shaped capacitive sensor disposed around its tip, which is connected with evaluation electronics. The evaluation electronics have an electrical oscillating circuit, the capacitor of which is formed by the sensor and the workpiece. When the torch, along with the sensor fixed in place on it, approaches the workpiece, the resonance frequency of the oscillating circuit decreases. The dependence of the resonance frequency on the distance between the sensor and the workpiece can be measured during a calibration measurement, so that the resonance frequency can serve as a measure for the distance between the sensor and the workpiece. This method has the disadvantage that the curve of the resonance frequency plotted over the distance can clearly shift due to ambient influences such as humidity or ambient temperature, so that setting the first location distance is imprecise when external conditions are changeable.